DESIGN: Searching for Explanations for Cryptogenic Stroke in the Young: Revealing the Etiology, Triggers, and Outcome (SECRETO) is an international prospective multicenter case-control study of young adults (age 18-49) presenting with an imaging-positive first-ever ischemic stroke of undetermined etiology (aim N=2000). Patients are included after standardized diagnostic procedures (brain MRI, imaging of intracranial and extracranial vessels, cardiac imaging, and screening for coagulopathies) and age- and sex-matched to healthy controls in a 1:1 fashion. Up to 45 study sites worldwide will be needed to recruit the planned participant population during a 3-year period. Neurovascular imaging and echocardiography studies, and ECGs will be read centrally.